Water heating apparatus and water heating system

ABSTRACT

In an immediate hot water supply operation mode in which a circulation pump is activated while a hot water supply faucet is closed, a water heating apparatus forms an immediate hot water supply circulation path by an inner path and an outer path as being combined, the inner path including a heating mechanism including a combustion mechanism and a heat exchanger, the outer path bypassing the hot water supply faucet outside the water heating apparatus. The outer path includes a crossover valve. In the immediate hot water supply operation mode, a controller introduces intermittent combustion to control a detection temperature detected by the temperature sensor to a set temperature. In intermittent combustion, a minimum combustion state and a combustion stop state are alternately provided. In the minimum combustion state, a quantity of heat output from the combustion mechanism is reduced to a minimum value.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to a water heating apparatus and a water heating system and more particularly to a water heating apparatus and a water heating system both with an immediate hot water supply function.

Description of the Background Art

A water heating apparatus of one form is equipped with what is called an immediate hot water supply function for outputting hot water at an appropriate temperature immediately after start of hot water supply even after hot water supply has been off for a long period of time. Normally, in order to achieve the immediate hot water supply function, a mode in which a circulation path that goes through a heat source also while hot water supply is off is formed (an “immediate hot water supply operation mode” below) should be provided.

U.S. Pat. No. 6,536,464 discloses a configuration for forming a circulation path for the immediate hot water supply function by externally connecting a bypass valve (which is also referred to as a “crossover valve” below) for thermostatic control using a wax thermostatic element. The immediate hot water supply function can thus be achieved by simplified attachment works without adding a function to control the crossover valve on a side of the water heating apparatus.

Japanese Patent Laying-Open No. 2015-230151 also describes a configuration in which an immediate hot water supply operation is performed by using a path to which a thermal valve similar to the crossover valve is connected.

SUMMARY OF THE INVENTION

In a circulation path to which a crossover valve (or a thermal valve) is connected, however, a pressure loss in a path that passes through a wax thermostatic element is high and hence a circulation flow rate is relatively low. Therefore, in the immediate hot water supply operation mode, control of fluid at a relatively low flow rate to a set temperature is required.

Therefore, in a water heating apparatus including a combustion mechanism such as a combustion burner as a heat source, there is a concern about difficulty in temperature control by adjustment of a quantity of heat generated by the combustion mechanism (an amount of fuel that is burnt).

The present disclosure was made to solve such problems, and an object of the present disclosure is to stabilize an operation with simplified temperature control in an immediate hot water supply operation mode by using a circulation path to which a crossover valve is connected.

According to one aspect of the present disclosure, a water heating apparatus that outputs hot water to a hot water supply faucet includes a heating mechanism including a combustion mechanism, a first temperature detector, a second temperature detector, a flow rate detector, and a controller. The water heating apparatus further includes an inner path. In an immediate hot water supply operation mode in which a circulation pump arranged inside or outside of the water heating apparatus is activated while the hot water supply faucet is closed, the inner path forms an immediate hot water supply circulation path through which fluid passes through the heating mechanism, as being combined with an outer path, the outer path bypassing the hot water supply faucet on the outside of the water heating apparatus. The outer path includes a thermal water stop bypass valve including a path that is closed when a temperature increases. The first temperature detector detects a fluid temperature upstream from the heating mechanism in the immediate hot water supply circulation path. The second temperature detector detects a fluid temperature downstream from the heating mechanism in the immediate hot water supply circulation path. The flow rate detector detects a circulation flow rate in the immediate hot water supply circulation path. The controller controls the heating mechanism and the circulation pump. The controller includes a heat quantity controller and a combustion controller. The heat quantity controller sets, in the immediate hot water supply operation mode, an output heat quantity command value for the combustion mechanism for controlling a temperature detection value detected by the second temperature detector to a set temperature in the immediate hot water supply operation mode. The combustion controller controls the combustion mechanism in accordance with the output heat quantity command value. The output heat quantity command value is set as being restricted within a range from a minimum heat quantity value to a maximum heat quantity value in a combustion state of the combustion mechanism. The combustion controller controls the combustion mechanism in the immediate hot water supply operation mode so as to alternately provide a minimum combustion state and a combustion stop state when the output heat quantity command value is set to the minimum heat quantity value and when the temperature detection value detected by the second temperature detector increases to a control upper limit temperature set to be higher than the set temperature. In the minimum combustion state, the combustion mechanism operates in accordance with the minimum heat quantity value.

According to another aspect of the present disclosure, a water heating system includes a water heating apparatus including a water entry port and a hot water output port, a low-temperature water pipe that introduces low-temperature water to the water entry port, a high-temperature water pipe that connects the hot water output port and a hot water supply faucet to each other, and a circulation pump arranged inside or outside the water heating apparatus. The water heating apparatus includes a heating mechanism including a combustion mechanism, a first temperature detector, a second temperature detector, a flow rate detector, and a controller. The water heating apparatus further includes an inner path. In an immediate hot water supply operation mode in which the circulation pump is activated while the hot water supply faucet is closed, the inner path forms an immediate hot water supply circulation path through which fluid passes through the heating mechanism, as being combined with an outer path, the outer path bypassing the hot water supply faucet on the outside of the water heating apparatus. The outer path includes a thermal water stop bypass valve including a path that is closed when a temperature increases. The first temperature detector detects a fluid temperature upstream from the heating mechanism in the immediate hot water supply circulation path. The second temperature detector detects a fluid temperature downstream from the heating mechanism in the immediate hot water supply circulation path. The flow rate detector detects a circulation flow rate in the immediate hot water supply circulation path. The controller controls the heating mechanism and the circulation pump. The controller includes a heat quantity controller and a combustion controller. The heat quantity controller sets, in the immediate hot water supply operation mode, an output heat quantity command value for the combustion mechanism for controlling a temperature detection value detected by the second temperature detector to a set temperature in the immediate hot water supply operation mode. The combustion controller controls the combustion mechanism in accordance with the output heat quantity command value. The output heat quantity command value is set as being restricted within a range from a minimum heat quantity value to a maximum heat quantity value in a combustion state of the combustion mechanism. The combustion controller controls the combustion mechanism in the immediate hot water supply operation mode so as to alternately provide a minimum combustion state and a combustion stop state when the output heat quantity command value is set to the minimum heat quantity value and when the temperature detection value detected by the second temperature detector increases to a control upper limit temperature set to be higher than the set temperature. In the minimum combustion state, the combustion mechanism operates in accordance with the minimum heat quantity value.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a water heating system including a water heating apparatus according to the present embodiment.

FIG. 2 is a block diagram illustrating an exemplary hardware configuration of a controller.

FIG. 3 shows a chart illustrating switching between flow paths by means of a crossover valve shown in FIG. 1.

FIG. 4 shows a state transition diagram involved with an immediate hot water supply operation by the water heating apparatus according to the present embodiment.

FIG. 5 is a block diagram illustrating temperature control in an immediate hot water supply operation mode.

FIG. 6 shows an exemplary operation waveform diagram of temperature control in the immediate hot water supply operation mode.

FIG. 7 is a flowchart illustrating control processing for determining whether or not a condition for deactivating the immediate hot water supply operation mode is satisfied.

FIG. 8 is a flowchart illustrating control processing for determining whether or not a condition for resuming the immediate hot water supply operation mode is satisfied.

FIG. 9 is a flowchart illustrating control processing in diagnosis as to an abnormal condition of an immediate hot water supply circulation path performed in the immediate hot water supply operation mode.

FIG. 10 is a block diagram illustrating a configuration of a water heating apparatus and a water heating system according to a modification of the present embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present disclosure will be described in detail below with reference to the drawings. The same or corresponding elements in the drawings below have the same reference characters allotted and description thereof will not be repeated in principle.

FIG. 1 is a block diagram illustrating a configuration of a water heating system 1A including a water heating apparatus according to the present embodiment.

Referring to FIG. 1, water heating system 1A includes a water heating apparatus 100, a low-temperature water pipe 110, a high-temperature water pipe 120, and a crossover valve 200. Water heating apparatus 100 includes a water entry port 11, a hot water output port 12, and a circulation port 13.

Low-temperature water pipe 110 is supplied with low-temperature water through a check valve 112. Low-temperature water is representatively supplied from a not-shown water supply pipe. Low-temperature water pipe 110 is connected to water entry port 11 and circulation port 13.

Water heating apparatus 100 includes a controller 10, a water entry path 20, a hot water output path 25, a circulation path 28, a bypass path 29, a combustion mechanism 30, a heat exchanger 40, a circulation pump 80, and a flow rate regulation valve 90.

Water entry path 20 is formed between water entry port 11 and an input side (upstream side) of heat exchanger 40 with a check valve 21 being interposed. Combustion mechanism 30 is representatively implemented by a burner that generates a quantity of heat by combustion of fuel such as gas or petroleum or the like.

Heat exchanger 40 increases a temperature of low-temperature water (fluid) introduced through water entry path 20 by using the quantity of heat generated by combustion mechanism 30. Combustion mechanism 30 and heat exchanger 40 implement an embodiment of the “heating mechanism.”

Hot water output path 25 is formed between an output side (downstream side) of heat exchanger 40 and hot water output port 12. Bypass path 29 connects water entry path 20 and hot water output path 25 to each other without heat exchanger 40 being interposed. Under the control of flow rate regulation valve 90 by controller 10, a ratio of a flow rate in bypass path 29 (a bypass flow rate ratio) to a total flow rate (the sum of a flow rate in heat exchanger 40 and a flow rate in bypass path 29) can be regulated.

According to such a bypass configuration, some of low-temperature water bypasses heat exchanger 40 and is mixed without being heated, in a portion downstream from heat exchanger 40, and thus high-temperature water is supplied from hot water output port 12. Since a temperature of output from heat exchanger 40 (heating mechanism) can thus be high, drainage water generated by cooling of exhaust from combustion mechanism 30 at a surface of heat exchanger 40 is advantageously suppressed.

A flow rate sensor 81 that outputs a value of a flow rate of low-temperature water is arranged in water entry path 20 and a flow rate sensor 82 is arranged in circulation path 28. Detection values from flow rate sensors 81 and 82 are input to controller 10. Flow rate sensor 81 is arranged to be included in an immediate hot water supply circulation path which will be described later.

A temperature sensor 71 is arranged in hot water output path 25 and a temperature sensor 73 is arranged in water entry path 20. A temperature sensor 72 is arranged in circulation path 28. Fluid temperatures detected by temperature sensors 71 to 73 are input to controller 10. A temperature sensor that detects a temperature of incogning water during a hot water supply operation is arranged also in water entry path 20. Temperature sensor 72 arranged upstream from heat exchanger 40 corresponds to an embodiment of the “first temperature detector” and temperature sensor 71 arranged downstream from heat exchanger 40 corresponds to an embodiment of the “second temperature detector.”

FIG. 2 is a block diagram illustrating an exemplary hardware configuration of controller 10.

Referring to FIG. 2, controller 10 is representatively implemented by a microcomputer. Controller 10 includes a central processing unit (CPU) 15, a memory 16, an input and output (I/O) circuit 17, and an electronic circuit 18. CPU 15, memory 16, and I/O circuit 17 can transmit and receive signals to one another through a bus 19. Electronic circuit 18 is configured to perform prescribed operation processing with dedicated hardware. Electronic circuit 18 can transmit and receive signals to and from CPU 15 and I/O circuit 17.

CPU 15 receives output signals (detection values) from sensors including temperature sensors 71 to 73 and flow rate sensors 81 and 82 through I/O circuit 17. CPU 15 further receives a signal indicating an operation instruction input to a remote controller 92 through I/O circuit 17. The operation instruction includes, for example, an operation to switch on and off an operation switch of water heating apparatus 100, a set hot water supply temperature, and various types of programmed time setting (which is also referred to as “timer setting”). CPU 15 controls operations by constituent apparatuses including combustion mechanism 30 and circulation pump 80 such that water heating apparatus 100 operates in accordance with the operation instruction.

CPU 15 can output visually or aurally recognizable information by controlling a notification apparatus 95. For example, notification apparatus 95 can output information by showing visually recognizable information such as characters and graphics on a screen. In this case, notification apparatus 95 can be implemented by a display screen provided in remote controller 92. Alternatively, notification apparatus 95 may be implemented by a speaker so that information can also be output by voice and immediate sound or melodies.

Operations by water heating apparatus 100 will be described with reference to FIG. 1 again.

In use for hot water supply in which a hot water supply faucet 330 is open, low-temperature water is introduced into water entry path 20 by a supply pressure of low-temperature water. When flow rate sensor 81 detects a flow rate exceeding a minimum operating quantity (MOQ) of working water while the operation switch of water heating apparatus 100 is on, controller 10 activates combustion mechanism 30.

Consequently, high-temperature water heated by combustion mechanism 30 and heat exchanger 40 is mixed with low-temperature water that passes through bypass path 29 and thereafter output from high-temperature water pipe 120 through hot water output port 12.

During a normal hot water supply operation, controller 10 deactivates circulation pump 80 and controls a temperature of fluid (hot water output temperature Th) detected by temperature sensor 71 to a set hot water supply temperature input to remote controller 92. Specifically, a temperature of hot water output can be controlled based on combination of control of a quantity of heating (a quantity of generated heat) by combustion mechanism 30 (heating mechanism) and control of the bypass flow rate ratio by means of flow rate regulation valve 90.

Circulation path 28 is formed between circulation port 13 and water entry path 20 (a connection point 22). Circulation pump 80 is connected to circulation path 28. Alternatively, circulation pump 80 may be connected to circulation port 13 on the outside of water heating apparatus 100. Activation and deactivation of circulation pump 80 are controlled by controller 10.

While the hot water supply operation is off, a temperature of fluid that remains in hot water output path 25 and high-temperature water pipe 120 is lowered. Therefore, there is a concern about a time period required until supply of high-temperature water to hot water supply faucet 330 after start of the next hot water supply operation. Therefore, water heating apparatus 100 is provided with an immediate hot water supply function for promptly supplying high-temperature water after start of the hot water supply operation. The immediate hot water supply function is performed by forming an immediate hot water supply circulation path including combustion mechanism 30 and heat exchanger 40 by activation of circulation pump 80 while the faucet is closed, that is, while hot water supply faucet 330 is closed.

For example, a user can designate by timer setting, a period for which the immediate hot water supply operation is to be performed. Timer setting can be input, for example, by operating remote controller 92. Alternatively, the period for which the immediate hot water supply operation is to be performed may automatically be set based on learning of a history of use by the user in the past. Alternatively, the period for which the immediate hot water supply operation is performed can also be started or ended directly in response to a switch operation by the user.

In water heating system 1A, the immediate hot water supply operation mode with activation of circulation pump 80 can be executed by using crossover valve 200. Crossover valve 200 is configured similarly to the thermostatically controlled bypass valve described in U.S. Pat. No. 6,536,464 and includes ports 201 to 204 and a wax thermostatic element 210. Ports 201 and 203 internally communicate with each other and ports 202 and 204 internally communicate with each other. Wax thermostatic element 210 is connected between ports 201 and 203 and ports 202 and 204.

Wax thermostatic element 210 forms a thermal bypass path between ports 201 and 203 and ports 202 and 204 in a low-temperature state. Wax thermostatic element 210 closes the thermal bypass path owing to thermal expansion force in a high-temperature state. A switching temperature at which switching between formation and closing of the thermal bypass path is made is designed in advance depending on a material and a configuration of wax thermostatic element 210. A state that a fluid temperature in crossover valve 200 is higher than the switching temperature is also referred to as a high-temperature state and a state that the fluid temperature is lower than the switching temperature is also referred to as a low-temperature state below.

Crossover valve 200 thus corresponds to an embodiment of the “thermal water stop bypass valve.” A pressure loss in the thermal bypass path is designed to be higher than a pressure loss in each of a path through which ports 201 and 203 communicate with each other and a path through which ports 202 and 204 communicate with each other.

Port 201 is connected to high-temperature water pipe 120 and port 202 is connected to low-temperature water pipe 110. Ports 203 and 204 are connected to hot water supply faucet 330. Hot water supply faucet 330 is provided as a combination faucet in which high-temperature water from port 203 and low-temperature water from port 204 are mixed. Valves 331 and 332 for adjustment of a ratio of mixing between high-temperature water and low-temperature water can be provided between port 204 and hot water supply faucet 330 and between port 203 and hot water supply faucet 330, respectively.

FIG. 3 shows a chart illustrating switching between flow paths by means of crossover valve 200 shown in FIG. 1.

Referring to FIGS. 3 and 1, while the faucet is open, that is, while paths from ports 203 and 204 to hot water supply faucet 330 are formed, due to the pressure loss described above, in each of the high-temperature state and the low-temperature state, a flow path Pa between high-temperature water pipe 120 and hot water supply faucet 330 and a flow path Pb between low-temperature water pipe 110 and hot water supply faucet 330 are formed.

While the faucet is closed, that is, while the paths from ports 203 and 204 to hot water supply faucet 330 are cut off, the flow path is switched between the low-temperature state and the high-temperature state. In the low-temperature state, a thermal bypass path Pc is formed between ports 201 and 202, that is, between high-temperature water pipe 120 and low-temperature water pipe 110, through a thermal bypass path formed in wax thermostatic element 210. In the high-temperature state, the thermal bypass path is closed so that the flow path between high-temperature water pipe 120 and low-temperature water pipe 110 is cut off.

In the hot water supply operation, in water heating system 1A, high-temperature water is obtained by heating of low-temperature water introduced into water entry port 11 through low-temperature water pipe 110 by combustion mechanism 30 and heat exchanger 40 (heating mechanism). High-temperature water is output from hot water supply faucet 330 through hot water output port 12 and high-temperature water pipe 120 as well as crossover valve 200 (flow path Pa).

In the immediate hot water supply operation mode, as circulation pump 80 is activated, a fluid path (outer path) from hot water output port 12 through high-temperature water pipe 120, crossover valve 200 (thermal bypass path Pc), and low-temperature water pipe 110 to circulation port 13 can be formed on the outside of water heating apparatus 100. In addition, in the inside of water heating apparatus 100, a fluid path (an inner path) including circulation port 13, circulation path 28, water entry path 20 (on the downstream side of connection point 22), heat exchanger 40 (heating mechanism), hot water output path 25, and hot water output port 12 can be formed. By forming the immediate hot water supply circulation path by the inner path and the outer path as such, high-temperature water flows through the immediate hot water supply circulation path also while the faucet is closed, so that high-temperature water can be supplied to hot water supply faucet 330 from immediately after the faucet is opened.

In the immediate hot water supply circulation path, temperature sensor 72 can detect a fluid temperature (a return temperature Tb) before heating and temperature sensor 71 can detect a fluid temperature (hot water output temperature Th) after heating.

A pressure loss in the thermal bypass path caused by wax thermostatic element 210 is high. Therefore, in consideration of a low flow rate in the immediate hot water supply circulation path including crossover valve 200, in water heating apparatus 100, flow rate regulation valve 90 is preferably controlled to maintain a bypass flow rate ratio r (0≤r<1.0) in the immediate hot water supply operation mode at a minimum value (including r=0 achieved in a fully closed state).

FIG. 4 shows a state transition diagram involved with the immediate hot water supply operation by water heating apparatus 100. State transition shown in FIG. 4 is controlled by controller 10.

Referring to FIG. 4, when a period for which the immediate hot water supply operation is to be performed designated by timer setting by a user is started, controller 10 has water heating apparatus 100 make transition from an “immediate hot water supply operation off mode” to an “immediate hot water supply operation on mode.”

When the hot water supply operation remains inactivate (the faucet is closed) and a temperature detected by temperature sensor 71 (a hot water output temperature) becomes lower than a predetermined mode reference temperature in the immediate hot water supply operation on mode, controller 10 determines that a start condition J0 is satisfied and activates circulation pump 80. The immediate hot water supply operation mode is thus started.

When a hot water supply interrupt condition J1 or a deactivation condition J2 which will be described later is satisfied in the immediate hot water supply operation mode, controller 10 deactivates circulation pump 80 and starts a stand-by mode. For example, the hot water supply interrupt condition is satisfied with increase in value of the flow rate detected by flow rate sensor 81.

When a resumption condition J3 which will be described later is satisfied in the stand-by mode, controller 10 quits the stand-by mode and resumes a circulation operation mode. When the period for which the immediate hot water supply operation is to be performed ends based on timer setting or in response to a switch operation in the stand-by mode, water heating apparatus 100 returns to the immediate hot water supply operation off mode. When the period for which the immediate hot water supply operation is to be performed ends in the immediate hot water supply operation mode, transition to the stand-by mode is made and thereafter water heating apparatus 100 returns to the immediate hot water supply operation off mode.

In the immediate hot water supply operation mode, by activating combustion mechanism 30 while the immediate hot water supply circulation path is formed by activation of circulation pump 80 with the faucet being closed, the fluid temperature in the immediate hot water supply circulation path can be increased. Therefore, the temperature in the immediate hot water supply circulation path is controlled by controlling an operation of combustion mechanism 30 in the immediate hot water supply operation mode.

In water heating system 1A, in crossover valve 200 included in the immediate hot water supply circulation path, a pressure loss in the thermal bypass path caused by wax thermostatic element 210 is high. Therefore, since the flow rate of circulating fluid that passes through heat exchanger 40 is low in the immediate hot water supply operation mode, an amount of increase in temperature of fluid with respect to a quantity of heat generated by combustion mechanism 30 is large. On the other hand, from a point of view of ensuring stable combustion, reduction in quantity of heat generated by combustion mechanism 30 is limited. Consequently, even though the quantity of heat generated by combustion mechanism 30 is controlled to the minimum value, there is a concern about excessive heating and resultant instability in temperature control.

Therefore, in the present embodiment, by controlling the combustion mechanism as will be described below, stable temperature control is achieved with simplified calculation.

FIG. 5 is a block diagram illustrating temperature control in the immediate hot water supply operation mode.

Referring to FIG. 5, controller 10 includes a heat quantity controller 10A and a combustion controller 10B. Functions of heat quantity controller 10A and combustion controller 10B can be achieved by software processing by controller 10 and/or hardware processing.

Heat quantity controller 10A calculates a command value (Pset) for a quantity of heat generated by combustion mechanism 30 for temperature control. In the water heating apparatus, the quantity of generated heat is generally calculated with a “scale number” being defined as a unit. The scale number=1 corresponds to a quantity of heat necessary for increasing the fluid temperature by 25° C. at a flow rate of 1 (L/minute). Therefore, the command value for the quantity of generated heat is also referred to as a scale number command value Pset below.

In the immediate hot water supply operation mode, a necessary amount of temperature increase ΔT is expressed as ΔT=Tr−Tb by using a detection temperature detected by temperature sensor 72 (return temperature Tb) and a set temperature Tr in the immediate hot water supply operation mode. For example, the quantity of heat generated by combustion mechanism 30 can be calculated in an expression (1) below in accordance with a product of a circulation flow rate Qt (L/minute) through the immediate hot water supply circulation path and amount of temperature increase ΔT. Set temperature Tr in the immediate hot water supply operation mode may be equal to or different from a set hot water supply temperature. Alternatively, set temperature Tr may be set to have a predetermined temperature difference from the set hot water supply temperature.

Pset=Qt×(Tr−Tb)/25  (1)

Circulation flow rate Qt can be detected by flow rate sensor 82. Alternatively, circulation flow rate Qt can be obtained also by multiplying a value of the flow rate (the flow rate in heat exchanger 40) detected by flow rate sensor 81 by 1/(1−r) time by using bypass flow rate ratio r. Namely, each of flow rate sensors 81 and 82 corresponds to an embodiment of the “flow rate detector” that detects a circulation flow rate.

In calculation of scale number command value Pset, actually, a ratio of a quantity of heat (thermal efficiency) used for temperature increase in heat exchanger 40 to a quantity of heat generated by combustion mechanism 30 should be taken into account. In the expression (1), however, for simplification of description, thermal efficiency is assumed as 1.0.

Heat quantity controller 10A sets scale number command value Pset as being restricted within a range from a smallest scale number Pmin to a largest scale number Pmax. When Pset calculated in accordance with the expression (1) is larger than Pmax (Pset>Pmax), the scale number command value is corrected to Pset=Pmax. Similarly, when Pset calculated in accordance with the expression (1) is smaller than Pmin (Pset<Pmin), the scale number command value is corrected to Pset=Pmin. Scale number command value Pset corresponds to the “output heat quantity command value,” smallest scale number Pmin corresponds to the “minimum heat quantity value,” and largest scale number Pmax corresponds to the “maximum heat quantity value.”

Heat quantity controller 10A can calculate scale number command value Pset also in common to that in the normal hot water supply operation, by substituting circulation flow rate Qt in the expression (1) with a value calculated by multiplying a flow rate detection value Q detected by flow rate sensor 81 by 1/(1−r) time described above, substituting the term of return temperature Tb with a temperature detection value (an incoming water temperature Tw) detected by temperature sensor 73, and substituting set temperature Tr with the set hot water supply temperature.

Combustion controller 10B generates an operation command value for combustion mechanism 30 based on scale number command value Pset from heat quantity controller 10A, a detection temperature detected by temperature sensor 71 (hot water output temperature Th), and set temperature Tr in the immediate hot water supply operation mode.

Combustion mechanism 30 includes a plurality of burners 31 a to 31 f, a proportional valve 34, and solenoid valves 36 to 38. Proportional valve 34 is disposed between a source fuel supply pipe 32 and a fuel supply pipe 33. A flow rate of fuel to be supplied to fuel supply pipe 33 can be controlled based on opening of proportional valve 34. A not-shown igniter is arranged in each of the plurality of burners 31 a to 31 f. The number of burners can be set to any number.

Solenoid valve 36 is connected between fuel supply pipe 33 and one burner 31 a. Solenoid valve 37 is connected between fuel supply pipe 33 and two burners 31 b and 31 c. Solenoid valve 38 is connected between fuel supply pipe 33 and three burners 31 d to 31 f. Combustion by burners 31 a to 31 f can be turned on and off by turning on and off solenoid valves 36 to 38. Therefore, the number of burners that burn fuel (which is also referred to as a combustion burner number Nbrn below) can be controlled based on combination of on and off commands for solenoid valves 36 to 38.

In the example in FIG. 5, when solenoid valves 36 to 38 are turned off, a condition of Nbm=0 is set and combustion mechanism 30 is in a combustion off state (or a combustion stop state). When solenoid valves 36 to 38 are all turned on, a condition of Nbrn=6 is set. By turning on at least one of solenoid valves 36 to 38, a condition of Nbm=1 to 5 can be set stepwise. In a combustion on state where at least one of solenoid valves 36 to 38 is turned on, a quantity of heat generated by combustion mechanism 30 is determined by combination between combustion burner number Nbrn and a flow rate of fuel.

Therefore, the operation command value for combustion mechanism 30 from combustion controller 10B includes an on and off command for solenoid valves 36 to 38 and an opening command value for proportional valve 34 in the example in FIG. 5.

Combustion controller 10B stores in advance a table that determines combination between the combustion burner number and the flow rate of fuel in correspondence with scale number command value Pset. Combustion controller 10B can generate, by referring to the table, an on and off command for solenoid valves 36 to 38 (a combustion burner number) and an opening command value for proportional valve 34 (a flow rate of fuel) for generating a quantity of heat in accordance with scale number command value Pset.

When combustion controller 10B controls combustion mechanism 30 into the combustion stop state, it generates an off command for all of solenoid valves 36 to 38. When flow rate detection value Q from flow rate sensor 81 is smaller than a minimum operating quantity MOQ, in order to deactivate combustion mechanism 30, an off command is generated for all of solenoid valves 36 to 38 and supply of fuel is also cut off.

FIG. 6 shows an exemplary operation waveform diagram of temperature control in the immediate hot water supply operation mode.

Referring to FIG. 6, combustion controller 10B controls on and off of combustion by combustion mechanism 30 based on comparison of a control upper limit temperature Trh and a control lower limit temperature Trl set in accordance with set temperature Tr with hot water output temperature Th (temperature sensor 71).

FIG. 6 shows a state in which circulation flow rate Qt is low, and therefore a state that a value calculated in accordance with the expression (1) is smaller than smallest scale number Pmin continues and the scale number command value calculated by heat quantity controller 10A is fixed to Pset=Pmin. In this case, even though combustion mechanism 30 is in a state that it outputs a quantity of heat in accordance with smallest scale number Pmin (which is also referred to as a “minimum combustion state” below) before time t1, hot water output temperature Th increases above set temperature Tr.

In the minimum combustion state in which a condition of Pset=Pmin is set, combustion controller 10B outputs an operation command to turn on solenoid valve 36 and turn off solenoid valves 37 and 38 in order to set the combustion burner number to Nbrn=1. Furthermore, combustion controller 10B outputs an opening command value for proportional valve 34 that corresponds to the lowest flow rate of fuel for stabilizing a combustion state of burner 31 a.

When hot water output temperature Th increases to control upper limit temperature Trh at time t1, at time t2 after lapse of a predetermined time period T1 (for example, approximately one second) since time t1, combustion controller 10B controls combustion mechanism 30 to a combustion stop state. In the combustion stop state, combustion controller 10B outputs an operation command to turn off solenoid valves 36 to 38.

Since the quantity of heat output from combustion mechanism 30 is 0 after time t2, hot water output temperature Th gradually lowers. When hot water output temperature Th lowers to a control lower limit temperature Trl at time t3, at time t4 after lapse of a predetermined time period T2 (for example, approximately one second) since time t3, combustion controller 10B controls combustion mechanism 30 to be in the combustion on state. In the combustion on state, the operation command for combustion mechanism 30 is generated to output a quantity of heat in accordance with scale number command value Pset from combustion mechanism 30. Similarly to the state before time t2, combustion controller 10B outputs the operation command for combustion mechanism 30 that corresponds to the minimum combustion state where the condition of Pset=Pmin is set.

Thus, after time t4 when combustion mechanism 30 is controlled to be in the minimum combustion state, similarly to the state before time t2, hot water output temperature Th increases. After hot water output temperature Th increases to control upper limit temperature Trh at time t5, combustion mechanism 30 is again controlled to be in the combustion stop state at time t6 after lapse of T1 since time t5.

Thus, in the water heating apparatus and the water heating system according to the present embodiment, by introducing intermittent combustion in which the minimum combustion state and the combustion stop state are alternately provided, even though the flow rate in the immediate hot water supply circulation path is low (representatively, an example where the condition of Pset=Pmin is set) as well, hot water output temperature Th can be controlled in a stable manner without excessively increasing. In particular, without making direct determination as to variation in flow rate caused by a behavior of wax thermostatic element 210, intermittent combustion can be controlled in a stable manner with simplified control based on scale number command value Pset that can be calculated in common to that in the normal hot water supply operation.

Though FIG. 6 illustrates a state that the scale number command value is fixed to Pset=Pmin, similar control is similarly applicable also to a state in which a condition of Pset>Pmin is set. Namely, even though the condition of Pset>Pmin is set, combustion mechanism 30 can be controlled to be in the combustion stop state in response to increase in hot water output temperature Th to control upper limit temperature Trh in the combustion on state of combustion mechanism 30, and combustion mechanism 30 can also be controlled to be in the combustion on state in accordance with scale number command value Pset in response to lowering in hot water output temperature Th to control lower limit temperature Trl in the combustion off state of combustion mechanism 30.

A condition (J2) for deactivating the immediate hot water supply operation mode and a condition (J3) for resuming the immediate hot water supply operation mode in the stand-by mode shown in FIG. 4 will now be described. Since the immediate hot water supply circulation path is formed or cut off by crossover valve 200 in accordance with a fluid temperature in water heating system 1A, the deactivation condition (J2) and the resumption condition (J3) should be set in consideration of this aspect. Though setting of the deactivation condition (J2) and the resumption condition (J3) which will be described below may be combined with intermittent combustion control described with reference to FIGS. 5 and 6, it is noted for a confirmation purpose that the setting can also be realized without being combined with intermittent combustion control.

FIG. 7 is a flowchart illustrating control processing for determining whether or not a condition for deactivating the immediate hot water supply operation mode is satisfied. Control processing shown in FIG. 7 is repeatedly performed by controller 10 in the immediate hot water supply operation mode.

Referring to FIG. 7, controller 10 determines in step (which is simply denoted as “S” below) 110 whether or not circulation flow rate Qt has lowered to a predetermined flow rate value (a first flow rate value). For example, in S110, when an MOQ off state in which flow rate detection value Q detected by flow rate sensor 81 is smaller than a minimum operating quantity (MOQ) continues for a certain time period (for example, two to three seconds), determination as YES is made. In this case, the minimum operating quantity (MOQ) in S110 corresponds to the “first flow rate value.”

In S120, controller 10 determines whether or not a temperature detection value detected by temperature sensor 72 (return temperature Tb) has increased. For example, when a state in which return temperature Tb is higher than a criterion temperature Tth1 continues for a certain time period (for example, approximately one to two seconds), increase in return temperature Tb is sensed and determination as YES is made in S120. Criterion temperature Tth1 in S120 corresponds to the “first criterion temperature.”

When controller 10 senses neither of lowering in circulation flow rate and increase in return temperature Tb (determination as NO in S110 and S120), controller 10 determines in S130 that the deactivation condition (J2) is not satisfied. Consequently, activation of circulation pump 80 is maintained and the immediate hot water supply operation mode is continued.

When controller 10 senses at least one of lowering in circulation flow rate and increase in return temperature Tb (determination as YES in S110 or S120), controller 10 determines in S140 that the condition (J2) for deactivating the immediate hot water supply operation mode is satisfied. When the deactivation condition (J2) is satisfied, circulation pump 80 is deactivated and transition from the immediate hot water supply operation mode to the stand-by mode is made in FIG. 4. In the stand-by mode, combustion mechanism 30 is also deactivated.

By setting the condition for deactivating the immediate hot water supply operation mode as shown in FIG. 7, activation of circulation pump 80 with the thermal bypass path within crossover valve 200 being closed with increase in fluid temperature in the immediate hot water supply circulation path can be prevented. By thus avoiding activation of circulation pump 80 with the immediate hot water supply circulation path being cut off, lifetime of circulation pump 80 can be prevented from becoming short.

In the stand-by mode in which combustion mechanism 30 and circulation pump 80 are inactive, fluid in the immediate hot water supply circulation path is stagnant and the fluid temperature gradually lowers. When the stand-by mode ends and the immediate hot water supply operation mode is resumed, in the immediate hot water supply circulation path including crossover valve 200, not only the temperature condition but also the state of the thermal bypass path within crossover valve 200 should be checked.

FIG. 8 is a flowchart illustrating control processing for determining whether or not the condition (J3) for resuming the immediate hot water supply operation mode is satisfied. Control processing shown in FIG. 8 is repeatedly performed by controller 10 in the stand-by mode.

Referring to FIG. 8, controller 10 determines in S210 whether or not a duration of the stand-by mode has attained to a value corresponding to a predetermined time period Tx (for example, approximately ten minutes). Count of the duration of a stand-by state is started at the time of transition from the immediate hot water supply operation mode to the stand-by mode.

When the duration of the stand-by mode attains to Tx (determination as YES in S210), controller 10 determines in S220 whether or not the fluid temperature in the immediate hot water supply circulation path has lowered.

In S220, when a state that the temperature detected by temperature sensor 71 or 72 (hot water output temperature Th or return temperature Tb) is lower than a criterion temperature Tth2 has continued for a certain time period (for example, approximately ten seconds), determination as YES can be made. For example, criterion temperature Tth2 can be set by subtracting a predetermined temperature γ (for example, γ is around 5° C.) from set temperature Tr in the immediate hot water supply operation mode (Tth2=Tr−γ). Criterion temperature Tth2 corresponds to the “second criterion temperature.”

Until the duration of the stand-by mode attains to Tx (determination as NO in S210) or when the fluid temperature in the immediate hot water supply circulation path has not lowered (determination as NO in S220), controller 10 allows the process to proceed to S270 and determines that the resumption condition (J3) is not satisfied. Consequently, the stand-by mode is continued and deactivation of circulation pump 80 and combustion mechanism 30 is maintained.

When the duration of the stand-by mode attains to Tx and the fluid temperature in the immediate hot water supply circulation path has lowered (determination as YES in S210 and S220), controller 10 activates circulation pump 80 in S230 and determines in S240 whether or not circulation flow rate Qt increases to a predetermined flow rate value (the second flow rate value). For example, in S240, the controller determines whether or not it detects MOQ on which represents a flow rate detection value detected by flow rate sensor 81 (or flow rate sensor 82) becoming higher than the minimum operating quantity (MOQ) within a certain time period (for example, approximately one minute) from activation of circulation pump 80 (S230).

When controller 10 does not detect MOQ on within the certain time period, controller 10 makes determination as NO in S240. In this case, in S260, count by the timer that counts the duration of the stand-by mode is cleared. Furthermore, in S270, the resumption condition (J3) is determined as not being satisfied, and the stand-by mode is continued. Thus, determination as NO in S210 is maintained and circulation pump 80 is not active until Tx (minutes) elapses again. Tx in S210 corresponds to the “first time period.”

When controller 10 detects MOQ on in response to activation of circulation pump 80, controller 10 makes determination as YES in S240 and controller 10 determines in S250 that the condition (J3) for resuming the immediate hot water supply operation mode is satisfied. When the resumption condition (J3) is satisfied, transition from the stand-by mode to the immediate hot water supply operation mode is made in FIG. 4 and hence activation of circulation pump 80 from S230 is maintained.

By setting the condition for resuming the immediate hot water supply operation mode as shown in FIG. 8, in activating circulation pump 80 in response to lowering in temperature of stagnant fluid, resumption of the immediate hot water supply operation mode in which circulation pump 80 is continuously activated with the thermal bypass path within crossover valve 200 being closed can be prevented. Thus, lifetime of circulation pump 80 can be prevented from becoming shorter.

By combining determination as to the duration of the stand-by mode in S210, the number of times of activation of circulation pump 80 with the thermal bypass path within crossover valve 200 being closed can be reduced.

In water heating system 1A, influence by the thermal bypass path within crossover valve 200 is preferably taken into consideration also in diagnosis as to an abnormal condition in the immediate hot water supply circulation path formed by activation of circulation pump 80.

FIG. 9 is a flowchart illustrating control processing in diagnosis as to an abnormal condition of the immediate hot water supply circulation path performed in the immediate hot water supply operation mode.

Referring to FIG. 9, when controller 10 senses in S310 transition from the immediate hot water supply operation off mode to the immediate hot water supply operation on mode shown in FIG. 4 (determination as YES in S310), the controller determines in S320 whether or not a predetermined time period Tc (for example, approximately five to six hours) has elapsed since stop of previous combustion by combustion mechanism 30. Determination as YES is made in S310 only at the time of start of the period for which the immediate hot water supply operation is to be performed as set by the timer or the like, and while the period for which the immediate hot water supply operation is to be performed continues, determination as NO is made. When controller 10 makes determination as NO in S310 or S320, in S315, the controller makes no determination as to an abnormal condition of the immediate hot water supply circulation path. Tc corresponds to the “second time period.”

Only when controller 10 makes determination as YES in both of S310 and S320, controller 10 allows the process to proceed to S330 and launches determination as to an abnormal condition of the immediate hot water supply circulation path.

When determination as to the abnormal condition is launched, controller 10 activates circulation pump 80 in S332. While circulation pump 80 is active, whether or not circulation flow rate Qt becomes higher than a diagnosis reference flow rate Qtst is determined in S334. In S334, circulation flow rate Qt based on a flow rate detection value detected by flow rate sensor 81 or 82 is compared with predetermined diagnosis reference flow rate Qtst.

When controller 10 does not detect circulation flow rate Qt exceeding diagnosis reference flow rate Qtst within a certain time period (for example, approximately one minute) from activation (S332) of circulation pump 80, controller 10 makes determination as NO in S334 and allows the process to proceed to S335. In S335, an abnormality count value Ncnt is increased by 1, and in S336, increased abnormality count value Ncnt is compared with a criterion value Nth set in advance.

When abnormality count value Ncnt attains to criterion value Nth (determination as YES in S336), controller 10 detects an abnormal condition of the immediate hot water supply circulation path in S338. In this case, notification apparatus 95 in FIG. 2 is used to notify a user of occurrence of the abnormal condition.

When circulation flow rate Qt exceeding diagnosis reference flow rate Qtst owing to activation of circulation pump 80 (S332) is detected (determination as YES in S334) or until abnormality count value Ncnt attains to criterion value Nth (determination as NO in S336), determination as to the abnormal condition of the immediate hot water supply circulation path ends in S339 without sensing the abnormal condition. In this case, processing in S320 or later is again performed in response to determination as YES in S310 at the time of start of the next period for which the immediate hot water supply operation is to be performed.

In diagnosis as to the abnormal condition of the immediate hot water supply circulation path shown in FIG. 9, only when crossover valve 200 is reliably in the low-temperature state and the thermal bypass path is formed, diagnosis as to the abnormal condition can be launched based on a result of determination in S310 and S320. Consequently, erroneous detection of the abnormal condition of the immediate hot water supply circulation path can be prevented. Unnecessary activation of circulation pump 80 with the thermal bypass path in crossover valve 200 being closed can also be avoided.

When the abnormal condition is not detected in S339 as a result of determination as YES in S334, abnormality count value Ncnt at that time point can also be cleared to the initial value (0). In this case, possibility that determination as NO in S334 sensed previously is a temporary phenomenon caused by clogging by a foreign matter or the like is taken into consideration.

Though diagnosis as to the abnormal condition of the immediate hot water supply circulation path shown in FIG. 9 may also be combined with intermittent combustion control described with reference to FIGS. 5 and 6 and/or a mode transition condition described with reference to FIGS. 7 and 8, it is noted for a confirmation purpose that diagnosis as to the abnormal condition can also be realized without being combined therewith.

A modification of the configuration of the water heating apparatus and the water heating system according to the present embodiment will now further be described.

FIG. 10 shows a block diagram illustrating a modification of the configuration of the water heating apparatus and the water heating system according to the modification of the present embodiment.

Referring to FIG. 10, a water heating system 1B includes a water heating apparatus 100X, low-temperature water pipe 110, high-temperature water pipe 120, and crossover valve 200. Water heating apparatus 100X includes water entry port 11 and hot water output port 12 without including circulation port 13. Therefore, unlike water heating apparatus 100 in FIG. 1, no circulation path 28 is provided in the inside of water heating apparatus 100X.

Low-temperature water pipe 110 supplied with low-temperature water through check valve 112 has a first end connected to water entry port 11 of water heating apparatus 100X and a second end connected to port 202 of crossover valve 200. Connection of crossover valve 200 to low-temperature water pipe 110, high-temperature water pipe 120, and hot water supply faucet 330 is the same as in water heating system 1A shown in FIG. 1. Circulation pump 80 is connected to water entry port 11.

In water heating system 1B, during the hot water supply operation, at least some of low-temperature water introduced from low-temperature water pipe 110 into water entry port 11 is heated by the heating mechanism (combustion mechanism 30 and heat exchanger 40). High-temperature water obtained by heating is output from hot water supply faucet 330 through hot water output port 12 and high-temperature water pipe 120 as well as crossover valve 200 (flow path Pa) as in water heating system 1A. Water heating apparatus 100X can thus also perform the hot water supply operation similarly to water heating apparatus 100.

In the immediate hot water supply operation mode, as circulation pump 80 is activated while the faucet is closed, a fluid path (outer path) from hot water output port 12 through high-temperature water pipe 120, crossover valve 200 (thermal bypass path Pc), and low-temperature water pipe 110 to water entry port 11 can be formed on the outside of water heating apparatus 100X. In addition, an inner path that passes through water entry port 11, water entry path 20, heat exchanger 40 (heating mechanism), hot water output path 25, and hot water output port 12 can be formed in the inside of water heating apparatus 100X as in FIG. 1. The immediate hot water supply circulation path can be formed by the inner path and the outer path also in water heating system 1B.

In water heating system 1B as well, circulation flow rate Qt in the immediate hot water supply circulation path can be detected by flow rate sensor 81 and return temperature Tb in the immediate hot water supply circulation path can be detected by temperature sensor 73. Therefore, also in water heating system 1B, as in water heating system 1A, intermittent combustion control described with reference to FIGS. 5 and 6 can be applied in the immediate hot water supply operation mode. Furthermore, also in water heating system 1B, transition between modes including the immediate hot water supply operation mode can be controlled in accordance with FIGS. 4, 7, and 8, and diagnosis as to an abnormal condition of the immediate hot water supply circulation path can be conducted in accordance with FIG. 9.

Crossover valve 200 described in U.S. Pat. No. 6,536,464 and shown in the present embodiment is merely an exemplary “thermal water stop bypass valve” and a valve containing a thermal bypass path of which formation and closing are switched depending on a temperature could be employed instead of crossover valve 200 in the present embodiment.

In water heating systems 1A and 1B, so long as the immediate hot water supply circulation path as above can be formed, circulation pump 80 can be arranged at any position on the outside or in the inside of water heating apparatus 100 without being limited to the configuration in the illustration in FIGS. 1 and 10. Even in such a configuration that circulation pump 80 is not contained in water heating apparatus 100, the immediate hot water supply operation mode described in the present embodiment can be realized by including controller 10 that controls deactivation and activation of circulation pump 80.

Though an example in which water heating apparatuses 100 and 100X each include a bypass configuration (bypass path 29 and flow rate regulation valve 90) is described in the present embodiment, the immediate hot water supply operation mode described in the present embodiment can be realized also in the configuration of water heating apparatuses 100 and 100X from which the bypass configuration is excluded.

Though an embodiment of the present invention has been described, it should be understood that the embodiment disclosed herein is illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 

What is claimed is:
 1. A water heating apparatus that outputs hot water to a hot water supply faucet, the water heating apparatus comprising: a heating mechanism including a combustion mechanism; an inner path, in an immediate hot water supply operation mode in which a circulation pump arranged inside or outside of the water heating apparatus is activated while the hot water supply faucet is closed, the inner path forming an immediate hot water supply circulation path through which fluid passes through the heating mechanism, as being combined with an outer path, the outer path bypassing the hot water supply faucet on outside of the water heating apparatus; a first temperature detector that detects a fluid temperature upstream from the heating mechanism in the immediate hot water supply circulation path; a second temperature detector that detects a fluid temperature downstream from the heating mechanism in the immediate hot water supply circulation path; a flow rate detector that detects a circulation flow rate in the immediate hot water supply circulation path; and a controller that controls the heating mechanism and the circulation pump, wherein the outer path includes a thermal water stop bypass valve including a path that is closed when a temperature increases, the controller includes a heat quantity controller that sets, in the immediate hot water supply operation mode, an output heat quantity command value for the combustion mechanism for controlling a temperature detection value detected by the second temperature detector to a set temperature in the immediate hot water supply operation mode, and a combustion controller that controls the combustion mechanism in accordance with the output heat quantity command value, the output heat quantity command value is set as being restricted within a range from a minimum heat quantity value to a maximum heat quantity value in a combustion state of the combustion mechanism, and the combustion controller controls the combustion mechanism in the immediate hot water supply operation mode so as to alternately provide a minimum combustion state and a combustion stop state when the output heat quantity command value is set to the minimum heat quantity value and when the temperature detection value detected by the second temperature detector increases to a control upper limit temperature set to be higher than the set temperature, in the minimum combustion state, the combustion mechanism operates in accordance with the minimum heat quantity value.
 2. The water heating apparatus according to claim 1, wherein at least in the immediate hot water supply operation mode, when the temperature detection value detected by the second temperature detector exceeds the control upper limit temperature, the combustion controller controls the combustion mechanism to be in the combustion stop state, and when the temperature detection value detected by the second temperature detector lowers to a control lower limit temperature set to be lower than the set temperature in the combustion stop state of the combustion mechanism, the combustion controller controls the combustion mechanism to be in the combustion state in accordance with the output heat quantity command value.
 3. The water heating apparatus according to claim 1, wherein in the immediate hot water supply operation mode, when the circulation flow rate becomes lower than a predetermined first flow rate value or when a detection temperature detected by the first temperature detector increases to a predetermined first criterion temperature, the controller deactivates the immediate hot water supply operation mode and starts a stand-by mode in which the circulation pump and the combustion mechanism are deactivated.
 4. The water heating apparatus according to claim 3, wherein in the stand-by mode, when the detection temperature detected by the first or second temperature detector lowers to a predetermined second criterion temperature, the controller activates the circulation pump, and when the circulation flow rate increases to a predetermined second flow rate value while the circulation pump is active, the controller quits the stand-by mode and resumes the immediate hot water supply operation mode.
 5. The water heating apparatus according to claim 4, wherein the controller does not activate the circulation pump until a duration of the stand-by mode reaches a predetermined first time period.
 6. The water heating apparatus according to claim 2, wherein in the immediate hot water supply operation mode, when the circulation flow rate becomes lower than a predetermined first flow rate value or when a detection temperature detected by the first temperature detector increases to a predetermined first criterion temperature, the controller deactivates the immediate hot water supply operation mode and starts a stand-by mode in which the circulation pump and the combustion mechanism are deactivated.
 7. The water heating apparatus according to claim 6, wherein in the stand-by mode, when the detection temperature detected by the first or second temperature detector lowers to a predetermined second criterion temperature, the controller activates the circulation pump, and when the circulation flow rate increases to a predetermined second flow rate value while the circulation pump is active, the controller quits the stand-by mode and resumes the immediate hot water supply operation mode.
 8. The water heating apparatus according to claim 7, wherein the controller does not activate the circulation pump until a duration of the stand-by mode reaches a predetermined first time period.
 9. The water heating apparatus according to claim 1, wherein in a mode on period during which execution of the immediate hot water supply operation mode is permitted, when the hot water supply faucet is closed and a detection temperature detected by the second temperature detector lowers to a predetermined mode reference temperature, the controller executes the immediate hot water supply operation mode, when a time period elapsed since stop of previous combustion by the combustion mechanism is longer than a predetermined second time period, the controller conducts diagnosis as to an abnormal condition of the immediate hot water supply circulation path, and in the diagnosis as to the abnormal condition, when the circulation flow rate does not increase to a predetermined diagnosis reference flow rate while the circulation pump is active, the abnormal condition of the immediate hot water supply circulation path is detected.
 10. The water heating apparatus according to claim 9, wherein when a phenomenon that the circulation flow rate does not increase to the diagnosis reference flow rate while the circulation pump is active is observed in each of a predetermined plurality of times of the diagnosis as to the abnormal condition, the controller detects the abnormal condition of the immediate hot water supply circulation path.
 11. A water heating system comprising: a water heating apparatus including a water entry port and a hot water output port; a low-temperature water pipe that introduces low-temperature water to the water entry port; a high-temperature water pipe that connects the hot water output port and a hot water supply faucet to each other; and a circulation pump arranged inside or outside the water heating apparatus, the water heating apparatus including a heating mechanism including a combustion mechanism, an inner path, in an immediate hot water supply operation mode in which the circulation pump is activated while the hot water supply faucet is closed, the inner path forming an immediate hot water supply circulation path through which fluid passes through the heating mechanism, as being combined with an outer path, the outer path bypassing the hot water supply faucet on outside of the water heating apparatus, a first temperature detector that detects a fluid temperature upstream from the heating mechanism in the immediate hot water supply circulation path, a second temperature detector that detects a fluid temperature downstream from the heating mechanism in the immediate hot water supply circulation path; a flow rate detector that detects a circulation flow rate in the immediate hot water supply circulation path, and a controller that controls the heating mechanism and the circulation pump, wherein the outer path includes a thermal water stop bypass valve including a path that is closed when a temperature increases, the controller includes a heat quantity controller that sets, in the immediate hot water supply operation mode, an output heat quantity command value for the combustion mechanism for controlling a temperature detection value detected by the second temperature detector to a set temperature in the immediate hot water supply operation mode, and a combustion controller that controls the combustion mechanism in accordance with the output heat quantity command value, the output heat quantity command value is set as being restricted within a range from a minimum heat quantity value to a maximum heat quantity value in a combustion state of the combustion mechanism, and the combustion controller controls the combustion mechanism in the immediate hot water supply operation mode so as to alternately provide a minimum combustion state and a combustion stop state when the output heat quantity command value is set to the minimum heat quantity value and when the temperature detection value detected by the second temperature detector increases to a control upper limit temperature set to be higher than the set temperature, in the minimum combustion state, the combustion mechanism operates in accordance with the minimum heat quantity value.
 12. The water heating system according to claim 11, wherein at least in the immediate hot water supply operation mode, when the temperature detection value detected by the second temperature detector exceeds the control upper limit temperature, the combustion controller controls the combustion mechanism to be in the combustion stop state, and when the temperature detection value detected by the second temperature detector lowers to a control lower limit temperature set to be lower than the set temperature in the combustion stop state of the combustion mechanism, the combustion controller controls the combustion mechanism to be in the combustion state in accordance with the output heat quantity command value.
 13. The water heating system according to claim 11, wherein in the immediate hot water supply operation mode, when the circulation flow rate becomes lower than a predetermined first flow rate value or when a detection temperature detected by the first temperature detector increases to a predetermined first criterion temperature, the controller deactivates the immediate hot water supply operation mode and starts a stand-by mode in which the circulation pump and the combustion mechanism are deactivated.
 14. The water heating system according to claim 12, wherein in the immediate hot water supply operation mode, when the circulation flow rate becomes lower than a predetermined first flow rate value or when a detection temperature detected by the first temperature detector increases to a predetermined first criterion temperature, the controller deactivates the immediate hot water supply operation mode and starts a stand-by mode in which the circulation pump and the combustion mechanism are deactivated. 